Photoelectrophoretic imaging with copper-free chlorophyll in the carrier liquid

ABSTRACT

A photoelectrophoretic imaging system in which high quality images are produced at high imaging speeds by the addition of copper-free chlorophyll to the imaging suspension.

Mar. 4, 1975 Unite States Patent Wells PHOTOELECTROPHORETIC IMAGING Chlorophyll C and A+B on lllumination Under Oxidiz- WITH COPPER-FREE CHLOROPHYLL 1N ing Conditions, Chem. Abstr., Vol 64, 1966, p. THE CARRIER LIQUID 5351f- .mu. .0 m, 9 Al W1 h 6 1 mo hV 7 s mb A f m e .lh C U W nm w CS my h M C A mm m in mm s a 9 Vn 0 S ,8 mm D. Wr. u X 0 r. 8 JX mn me: s mA s sus 77 ii Conn 22 Filed: Dec. 14, 1973 21 Appl. No.: 425,015

Primary E.\'aminerNorman G. T orchin Assistant E.\'aminer-Judson R. Hightower Attorney, Agent, or Firm-James J. Ralabate; David C.

T C A m .m S l B .i n A O T A d r a h .m R m It 5 P [l i i .O2 O //3 621 7. 95 iz u2 3 0 .lR R P 0 O 900 00 H m H E4 %4 P0 0 1 m m 6 n" 9 "n m In H c Hr. "a u .9 u M I C l d S ind U ImF w N 5 A photoelectrophoretic imaging system in which high References Cited UNITED STATES PATENTS quality images are produced at high imaging speeds by the addition of copper-free chlorophyll to the imaging suspension.

3,615.558 Carreira.....I........................ 96/1 PE 7 Claims, 1 Drawing Figure OTHER PUBLICATIONS "Change of Electrical Conductivity of Solutions of PHOTOELECTROPHORETIC IMAGING WITH COPPER-FREE CHLOROPI-IYLL IN THE CARRIER LIQUID BACKGROUND OF THE INVENTION This invention relates to imaging systems and, more specifically, to photoelectrophoretic imaging.

In photoelectrophoretic imaging finely divided particles of electrically photosensitive materials are dispersed in an electrically insulating liquid. The imaging suspension thus formed is placed between electrodes. The particles which are believed to bear an initial charge as a result of being suspended in the carrier liquid on application of an electrical field are drawn toward an electrode. The imaging suspension is then exposed to a light image. The particles within interaction range of the electrode to which they are drawn are struck by light to which they are responsive and, being electrically photosensitive, exchange charge with the electrode and are repelled by it leaving behind a positive image of particles not struck by light to which they are responsive and forming a negative image on the opposing electrode. The process may be used to form a monochromatic or polychromatic image. A detailed discussion of operating conditions, apparatus and materials used in photoelectrophoretic imaging may be found in U.S. Pat. Nos. 3,383,933; 3,384,488; 3,384,565 and 3,384,566, all issued May 2], 1968, the complete disclosures of which are incorporated herein by reference.

As with any imaging system, it is desirable to increase the rate at which images may be formed. One of the difficulties which arises when imaging speeds are increased in photoelectrophoretic imaging is that the density of the image drops off and background increases to reduce image contrast. There is, thus, a continuous need for increasing the speed at which a photoelectrophoretic imaging system will produce high quality images.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a photoelectrophoretic imaging system which overcomes the above noted disadvantages.

lt is another object of this invention to provide a relatively high contrast imaging system.

It is another object ofthis invention to provide a relatively high speed photoelectrophoretic imaging system.

The above objects and others are accomplished in accordance with the present invention by providing an imaging suspension for photoelectrophoretic imaging which contains, in addition to the electrically photosensitive material, a sensitizing amount of copper-free chlorophyll. The imaging suspension is positioned between two electrodes and exposed to an imagewise pattern of electromagnetic radiation containing wavelengths of radiation to which at least a portion of the electrically photosensitive particles are responsive. The combination ofelectric field and radiation causes particle migration in image configuration.

In a preferred embodiment the imaging suspension is formed in a layer on a transparent conductive electrode called the injecting electrode. A second electrode having an insulating outer surface and call the blocking electrode is then brought into contact with the free surface oftheimaging suspension while a high d.c. electrical potential difference is applied between the conductive surface of the injecting electrode and the conductive core of the blocking electrode. The polarity of the field is selected to draw a majority of the electri cally photosensitive particles to the transparent conductive electrode through which imagewise exposure is made. On imagewise exposure particles, struck by radiation to which they are sensitive, exchange charge with the injecting electrode and adhere to the insulating surface of the blocking electrode leaving a positive image on the surface of the transparent conductive electrode made up of particles not struck by light to which they are sensitive. The particles adhering to the blocking electrode are less able to exchange charge with the insulating surface and remain there forming a negative image.

It can be seen from the above description that the density of both the positive and the negative image depends on the amount of particles which initially are drawn to the injecting electrode. This, in turn, depends on the concentration of particles in the suspension and the strength and sign of the charge on the particles in the suspension. If some particles have the wrong sign of charge or little or no charge, they do not as effectively enter into the image forming step. A particularly preferred embodiment of the present invention utilizes a precharge step to force the particles to take on a high charge of the desired polarity. This may be done by, e.g., corona charging the suspension prior to imaging to electrophoretically deposit the particles. Another technique is shown in U.S. Pat. No. 3,645,874 to J. B. Wells, issued Feb. 29, 1972, the complete disclosure of which is hereby incorporated by reference. This patent discloses contacting the free surface of the imaging suspension with an electrode and applying an electrical field across the suspension to electrophoretically deposit the particles. Electrophoretic deposition increases the speed at which quality images can be formed. The addition of copper-free chlorophyll to the imaging suspension increases the speed at which qual ity images can be formed whether electrophoretic predeposition is used or not.

The insulating carrier liquid for the imaging suspension of the present invention may be of any suitable material. Typical insulating materials include dec ane, dodecane, tetradecane, kerosene, molten paraffin, molten beeswax or other molten thermoplastic material, mineral oil,'silicone oils such as dimethyl polysiloxane, fluorinated hydrocarbons and mixtures thereof.

The electrically photosensitive material may be organic or inorganic.

Typical organic materials include pigments such as quinacridones, carboxamides, carboxanalides, triazines, benzopyraocolines, anthraquinones, azos, salts and lakes of compounds derived from 9- phenylxanthene, dioxazines, lakes of fluorescein dyes, substituted pyrene, bisazos, phthalocyanines and mixtures thereof. Specific organic materials are listed at Columns 8 and 9 ofU.S. Pat. No. 3,384,488 issued May 21, 1968.

Further organic and inorganic materials include those listed at Column 9, line 67 through Column l 1, line 1 of the above patent. Other inorganic materials include cadmium sulfide, cadmium sulfoselenide, zinc oxide, zinc sulfide, sulphur, selenium, mercuric sulfide, lead oxide, lead sulfide, cadmium selenide, titanium dioxide, indium trioxide and mixtures thereof. Mixtures of inorganic and organic materials may also be used.

BRIEF DESCRIPTION OF THE DRAWING The advantages of this improved method of photoelectrophoretic imaging will become apparent upon consideration of the detailed disclosure of the invention particularly when considered in conjunction with the accompanying drawing which shows a simplified schematic cross-section of a continuous photoelectrophoretic imaging system.

Referring now to the drawing, there is shown transparent conductive injecting electrode generally designated l, which in this exemplary instance is tin oxide 3 coated on glass cylinder 2. A uniform layer of imaging suspension 5 from sump 7 is coated by applicator 6 onto electrode 1. To ensure that a uniform coating is applied, roller 8 may be utilized.

Electrodes 40 and 20 may be used to electrophoretically deposit particles on surface 3 and remove excess carrier liquid. Metering device 26 may alternatively be used. Blocking electrode generally designated 10 in this exemplary instance has a conductive core 11 surrounded by insulating material 12. A receiver sheet 13 which may be paper is used to receive images.

Electrodes 10, and 40 preferably have an outer insulating layer designated as layers 12, 22 and 42, respectively. Although not required, these layers are preferred to help support the high fields and in the case of electrode 10 prevent particle oscillation in the system.

In operation suspension 5 is coated on electrode 1. Electrical field is applied across suspension 5 by electrodes 40 and 20 to deposit particles on surface 3. The suspension is exposed to a light image reflected off of mirror 39 in the area of closest approach between electrodes l and 10 with field applied. Light struck particles move to receiver sheet 13 and form a negative image thereon which may be fixed on receiver 13 using, e.g., heat source 38 or the image may be transferred. By utilizing in the imaging suspension copper-free chlorophyll. acceptable quality images have been'produced at a rate of 108 inches per second with legible images being produced at a rate of 155 inches per second.

DESCRIPTION OF PREFERRED EMBODIMENTS The following Examples further specifically illustrate the improved photoelectrophoretic imaging system of the present invention. Parts and percentages are by weight unless otherwise stated.

In the following Examples, the NESA electrode consists of a 6 inch diameter Pyrex glass cylinder concentric to about 0.001 inch with a conductive tin oxide coating. The blocking electrode consists of a 4 inch diameter conductive steel core with a A inch thick layer of polyurethane on its surface. Paper is driven between the blocking electrode and the suspension to receive the images. Electrodes 40 and 20 in the drawing are /2 inch diameter aluminum cores covered with inch polyurethane sleeves.

EXAMPLE I PRIOR ART An imaging suspension is provided by dispersing about 5 grams of the X-form of metal-free phthalocyanine made as shown in U.S. Pat. No. Re. 27,117, in about 100 cc Sohio Odorless Solvent 3454, a mixture of kerosene fractions. The suspension is ball-milled until the diameter of the phthalocyanine particles is below about one micron. The suspension is coated to a thickness of about 4 microns onto the conductive surface of the glass cylinder using a urethane roller. The film of imaging suspension is metered to a thickness of about 3 microns as it passes beneath electrode 40 with the electrode 40 held at a potential of about 1,000 volts with respect to the conductive surface of the glass cylinder which is connected to ground. The suspension then passes under roller 20 with a potential of about +7,000 volts with respect to the grounded conductive surface of the glass cylinder being applied. The film of suspension is then exposed to a light image at the nip formed by the glass cylinder and blocking electrode with the blocking electrode being held at a potential of about +8,000 volts with respect to the conductive surface of the glass cylinder. The source ofthe light image is light projected through a black and white transparency using a 500 watt quartz iodine lamp. The images are formed on the paper web at a rate of about inches per second. A positive cyan image is formed on the conductive surfacev of the glass cylinder and a negative image is formed on the paper web. By positive image is meant that the particles of cyan pigment are absent from light struck areas but remain behind in non-light struck areas. Images are formed with a Dmin of about 0.68. The images were barely discernible.

EXAMPLE II The experiment of Example I is repeated except that approximately 5 gramsof a 4% solution of copper-free chlorophyll in cottonseed oil available from Dayco Labs, Inc., Kansas City, is added to the suspension. At an imaging speed of about 110 inches per second, the Dmax is about 0.29 and the Dmin about 0.01. The imaging speed could be increased to about inches per second before the background Dmin becomes objectionable.

EXAMPLES III PRIOR ART A suspension is prepared to provide black images. Black particles are made by combining in a single particle particles of a yellow pigment, a cyan pigment and a magenta pigment, about 24 grams of Irgazine red as described in U.S. Pat. No. 2,973,358 available from Geigy Chemical Corp., 18 grams of Algol Yellow (l,2,5,6-di(C,C'-diphenyl) thiazoleanthraquinone) available from General Aniline & Film Corp. and about 30 grams of the X form of phthalocyanine as described in U.S. Pat. No. Re. 27,117 in finely divided form are sonified in methyl ethyl ketone, combined with about 72 grams of a 35/65 copolymer of n-butylmethacrylate and styrene and spray dried.

The particles thus formed have a particle size of about 7 microns. About 16 grams of the composite particle thus formed are dispersed in about 400 cc of Sohio 3454. The suspension thus formed is used as in Example 1. Images are formed at a rate of about 15 inches per second. Dmax was found to be 0.70, Dmin greater than 0.10.

EXAMPLE IV The experiment of Example III was repeated except that about 2 grams ofthe chlorophyll of Example II was added to the suspension. At an imaging speed of about 15 inches per second, the Dmax is about 1.10 and the Dmin is less than about 0.01.

Although specific components and proportions have been described in the above Examples, other materials as listed above where suitable may be used with similar results. In addition other materials may be added to the various layers to synergize, enhance or otherwise modify their properties. For example, a wax-like material may be added to the suspension which on evaporation of the liquid will aid fixing.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention. The effective concentration of copper-free chlorophyll ranges from about 0.1 to about 2 weight percent based on the total weight of the suspension.

What is claimed is:

1. A method of imaging which comprises:

a. providing a layer of a suspension of electrically photosensitive particles in an insulating carrier liquid on a first electrode, said imaging suspension also containing copper-free chlorophyll;

b. exposing said suspension to a pattern of electromagnetic radiation to which at least a portion of said particles are responsive, and

c. applying an electrical field across said suspension until an image is formed.

2. The method of claim 1 wherein said copper-free chlorophyll is present in a range of from about 0.1% to about 2% parts by weight based on the total weight of the suspension.

3. The method of claim 1 including the additional steps of: I

prior to step (b) contacting said suspension with a roller electrode while applying a potential difference between said roller electrode and said first electrode sufficient to deposit the particles on said first electrode and subsequently contacting said suspension with a second roller electrode while applying a potential difference of at least about 5,000 volts across said suspension.

4. The method of claim 1 wherein said particles comprise metal-free phthalocyanine in the Xform.

5. The method of claim 1 wherein said particles comprise a composite particle comprising yellow particles, magenta particles and cyan particles associated with a resin.

6. The method of claim 5 wherein said cyan particle comprises metal-free phthalocyanine in the X-form.

7. The method of claim 5 wherein said resin comprises a copolymer of n-butyl-methacrylate and styrene. 

1. A METHOD OF IMAGING WHICH COMPRISES: A. PROVIDING A LAYER OF A SUSPENSION OF ELECTRICALLY PHOTOSENSITIVE PARTICLES IN AN INSULATING CARRIER LIQUID ON A FIRST ELECTRODE, SAID IMAGING SUSPENSION ALSO CONTAINING COPPER-FREE CHLORPHYLL; B. EXPOSING SAID SUSPENSION TO A PATTERN OF ELECTROMAGNETIC RADIATION TO WHICH AT LEAST A PORTION OF SAID PARTICLES ARE RESPONSIVE, AND
 2. The method of claim 1 wherein said copper-free chlorophyll is present in a range of from about 0.1% to about 2% parts by weight based on the total weight of the suspension.
 3. The method of claim 1 including the additional Steps of: prior to step (b) contacting said suspension with a roller electrode while applying a potential difference between said roller electrode and said first electrode sufficient to deposit the particles on said first electrode and subsequently contacting said suspension with a second roller electrode while applying a potential difference of at least about 5,000 volts across said suspension.
 4. The method of claim 1 wherein said particles comprise metal-free phthalocyanine in the X-form.
 5. The method of claim 1 wherein said particles comprise a composite particle comprising yellow particles, magenta particles and cyan particles associated with a resin.
 6. The method of claim 5 wherein said cyan particle comprises metal-free phthalocyanine in the X-form.
 7. The method of claim 5 wherein said resin comprises a copolymer of n-butyl-methacrylate and styrene. 